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Message-Id: <C2FODELKAJ5Q.2PTDHBKV4JEY8@geist>
Date: Fri, 01 May 2020 15:28:39 -0500
From: "Christopher M. Riedl" <cmr@...ormatik.wtf>
To: "Christophe Leroy" <christophe.leroy@....fr>,
<linuxppc-dev@...ts.ozlabs.org>, <kernel-hardening@...ts.openwall.com>
Subject: Re: [RFC PATCH v2 3/5] powerpc/lib: Use a temporary mm for code
patching
On Wed Apr 29, 2020 at 7:52 AM, Christophe Leroy wrote:
>
>
>
>
> Le 29/04/2020 à 04:05, Christopher M. Riedl a écrit :
> > Currently, code patching a STRICT_KERNEL_RWX exposes the temporary
> > mappings to other CPUs. These mappings should be kept local to the CPU
> > doing the patching. Use the pre-initialized temporary mm and patching
> > address for this purpose. Also add a check after patching to ensure the
> > patch succeeded.
> >
> > Use the KUAP functions on non-BOOKS3_64 platforms since the temporary
> > mapping for patching uses a userspace address (to keep the mapping
> > local). On BOOKS3_64 platforms hash does not implement KUAP and on radix
> > the use of PAGE_KERNEL sets EAA[0] for the PTE which means the AMR
> > (KUAP) protection is ignored (see PowerISA v3.0b, Fig, 35).
> >
> > Based on x86 implementation:
> >
> > commit b3fd8e83ada0
> > ("x86/alternatives: Use temporary mm for text poking")
> >
> > Signed-off-by: Christopher M. Riedl <cmr@...ormatik.wtf>
> > ---
> > arch/powerpc/lib/code-patching.c | 149 ++++++++++++-------------------
> > 1 file changed, 55 insertions(+), 94 deletions(-)
> >
> > diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-patching.c
> > index 259c19480a85..26f06cdb5d7e 100644
> > --- a/arch/powerpc/lib/code-patching.c
> > +++ b/arch/powerpc/lib/code-patching.c
> > @@ -19,6 +19,7 @@
> > #include <asm/page.h>
> > #include <asm/code-patching.h>
> > #include <asm/setup.h>
> > +#include <asm/mmu_context.h>
> >
> > static int __patch_instruction(unsigned int *exec_addr, unsigned int instr,
> > unsigned int *patch_addr)
> > @@ -72,101 +73,58 @@ void __init poking_init(void)
> > pte_unmap_unlock(ptep, ptl);
> > }
> >
> > -static DEFINE_PER_CPU(struct vm_struct *, text_poke_area);
> > -
> > -static int text_area_cpu_up(unsigned int cpu)
> > -{
> > - struct vm_struct *area;
> > -
> > - area = get_vm_area(PAGE_SIZE, VM_ALLOC);
> > - if (!area) {
> > - WARN_ONCE(1, "Failed to create text area for cpu %d\n",
> > - cpu);
> > - return -1;
> > - }
> > - this_cpu_write(text_poke_area, area);
> > -
> > - return 0;
> > -}
> > -
> > -static int text_area_cpu_down(unsigned int cpu)
> > -{
> > - free_vm_area(this_cpu_read(text_poke_area));
> > - return 0;
> > -}
> > -
> > -/*
> > - * Run as a late init call. This allows all the boot time patching to be done
> > - * simply by patching the code, and then we're called here prior to
> > - * mark_rodata_ro(), which happens after all init calls are run. Although
> > - * BUG_ON() is rude, in this case it should only happen if ENOMEM, and we judge
> > - * it as being preferable to a kernel that will crash later when someone tries
> > - * to use patch_instruction().
> > - */
> > -static int __init setup_text_poke_area(void)
> > -{
> > - BUG_ON(!cpuhp_setup_state(CPUHP_AP_ONLINE_DYN,
> > - "powerpc/text_poke:online", text_area_cpu_up,
> > - text_area_cpu_down));
> > -
> > - return 0;
> > -}
> > -late_initcall(setup_text_poke_area);
> > +struct patch_mapping {
> > + spinlock_t *ptl; /* for protecting pte table */
> > + pte_t *ptep;
> > + struct temp_mm temp_mm;
> > +};
> >
> > /*
> > * This can be called for kernel text or a module.
> > */
> > -static int map_patch_area(void *addr, unsigned long text_poke_addr)
> > +static int map_patch(const void *addr, struct patch_mapping *patch_mapping)
> > {
> > - unsigned long pfn;
> > - int err;
> > + struct page *page;
> > + pte_t pte;
> > + pgprot_t pgprot;
> >
> > if (is_vmalloc_addr(addr))
> > - pfn = vmalloc_to_pfn(addr);
> > + page = vmalloc_to_page(addr);
> > else
> > - pfn = __pa_symbol(addr) >> PAGE_SHIFT;
> > + page = virt_to_page(addr);
> >
> > - err = map_kernel_page(text_poke_addr, (pfn << PAGE_SHIFT), PAGE_KERNEL);
> > + if (radix_enabled())
> > + pgprot = PAGE_KERNEL;
> > + else
> > + pgprot = PAGE_SHARED;
> >
> > - pr_devel("Mapped addr %lx with pfn %lx:%d\n", text_poke_addr, pfn, err);
> > - if (err)
> > + patch_mapping->ptep = get_locked_pte(patching_mm, patching_addr,
> > + &patch_mapping->ptl);
> > + if (unlikely(!patch_mapping->ptep)) {
> > + pr_warn("map patch: failed to allocate pte for patching\n");
> > return -1;
> > + }
> > +
> > + pte = mk_pte(page, pgprot);
> > + if (!IS_ENABLED(CONFIG_PPC_BOOK3S_64))
> > + pte = pte_mkdirty(pte);
>
>
> Why only when CONFIG_PPC_BOOK3S_64 is not set ?
>
>
> PAGE_KERNEL should already be dirty, so making it dirty all the time
> shouldn't hurt.
>
>
Ok, I'll remove this check to simplify.
> > + set_pte_at(patching_mm, patching_addr, patch_mapping->ptep, pte);
> > +
> > + init_temp_mm(&patch_mapping->temp_mm, patching_mm);
> > + use_temporary_mm(&patch_mapping->temp_mm);
> >
> > return 0;
> > }
> >
> > -static inline int unmap_patch_area(unsigned long addr)
> > +static void unmap_patch(struct patch_mapping *patch_mapping)
> > {
> > - pte_t *ptep;
> > - pmd_t *pmdp;
> > - pud_t *pudp;
> > - pgd_t *pgdp;
> > -
> > - pgdp = pgd_offset_k(addr);
> > - if (unlikely(!pgdp))
> > - return -EINVAL;
> > -
> > - pudp = pud_offset(pgdp, addr);
> > - if (unlikely(!pudp))
> > - return -EINVAL;
> > -
> > - pmdp = pmd_offset(pudp, addr);
> > - if (unlikely(!pmdp))
> > - return -EINVAL;
> > -
> > - ptep = pte_offset_kernel(pmdp, addr);
> > - if (unlikely(!ptep))
> > - return -EINVAL;
> > + /* In hash, pte_clear flushes the tlb */
> > + pte_clear(patching_mm, patching_addr, patch_mapping->ptep);
> > + unuse_temporary_mm(&patch_mapping->temp_mm);
> >
> > - pr_devel("clearing mm %p, pte %p, addr %lx\n", &init_mm, ptep, addr);
> > -
> > - /*
> > - * In hash, pte_clear flushes the tlb, in radix, we have to
> > - */
> > - pte_clear(&init_mm, addr, ptep);
> > - flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
> > -
> > - return 0;
> > + /* In radix, we have to explicitly flush the tlb (no-op in hash) */
> > + local_flush_tlb_mm(patching_mm);
> > + pte_unmap_unlock(patch_mapping->ptep, patch_mapping->ptl);
> > }
> >
> > static int do_patch_instruction(unsigned int *addr, unsigned int instr)
> > @@ -174,33 +132,36 @@ static int do_patch_instruction(unsigned int *addr, unsigned int instr)
> > int err;
> > unsigned int *patch_addr = NULL;
> > unsigned long flags;
> > - unsigned long text_poke_addr;
> > - unsigned long kaddr = (unsigned long)addr;
> > + struct patch_mapping patch_mapping;
> >
> > /*
> > - * During early early boot patch_instruction is called
> > - * when text_poke_area is not ready, but we still need
> > - * to allow patching. We just do the plain old patching
> > + * The patching_mm is initialized before calling mark_rodata_ro. Prior
> > + * to this, patch_instruction is called when we don't have (and don't
> > + * need) the patching_mm so just do plain old patching.
> > */
> > - if (!this_cpu_read(text_poke_area))
> > + if (!patching_mm)
> > return raw_patch_instruction(addr, instr);
> >
> > local_irq_save(flags);
> >
> > - text_poke_addr = (unsigned long)__this_cpu_read(text_poke_area)->addr;
> > - if (map_patch_area(addr, text_poke_addr)) {
> > - err = -1;
> > + err = map_patch(addr, &patch_mapping);
> > + if (err)
> > goto out;
> > - }
> >
> > - patch_addr = (unsigned int *)(text_poke_addr) +
> > - ((kaddr & ~PAGE_MASK) / sizeof(unsigned int));
> > + patch_addr = (unsigned int *)(patching_addr | offset_in_page(addr));
> >
> > - __patch_instruction(addr, instr, patch_addr);
> > + if (!radix_enabled())
> > + allow_write_to_user(patch_addr, sizeof(instr));
> > + err = __patch_instruction(addr, instr, patch_addr);
> > + if (!radix_enabled())
> > + prevent_write_to_user(patch_addr, sizeof(instr));
> >
> > - err = unmap_patch_area(text_poke_addr);
> > - if (err)
> > - pr_warn("failed to unmap %lx\n", text_poke_addr);
> > + unmap_patch(&patch_mapping);
> > + /*
> > + * Something is wrong if what we just wrote doesn't match what we
> > + * think we just wrote.
> > + */
> > + WARN_ON(*addr != instr);
> >
> > out:
> > local_irq_restore(flags);
> >
>
>
> Christophe
>
>
>
>
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